1. Technical Field
The present disclosure relates to an organic light emitting diode display device, and more particularly, to an organic light emitting diode display device including a polymeric nanoparticle and a method of fabricating the organic light emitting diode display device.
2. Discussion of Related Art
Among various flat panel displays (FPDs), an organic light emitting diode (OLED) display device has superior properties such as high luminance and low driving voltage. The OLED display device uses an emissive electroluminescent layer to realize a high contrast ratio and a thin profile, and is excellent at displaying a moving image because of a short response time of several micro seconds (μsec). Also, the OLED display device has no limitation on a viewing angle and is stable even in a low temperature. Since the OLED display device is typically driven by a low voltage of about 5V to about 15V in direct current (DC), fabrication and design of a driving circuit is easy. Further, a fabrication process for the OLED display device including a deposition and an encapsulation is simple.
The OLED display device may display an image using light emitted from a light emitting diode in each pixel region. The light emitting diode of the OLED display device will be illustrated hereinafter.
FIG. 1 is a cross-sectional view showing an organic light emitting diode display device according to a related art.
In FIG. 1, a light emitting diode ED is formed in each pixel region of an organic light emitting diode (OLED) display device 10 according to the related art. The light emitting diode ED includes a first electrode 34, an emission layer 38 and a second electrode 40.
Although not shown, the light emitting diode ED is connected to a thin film transistor (TFT) on a first substrate, and a passivation layer 42 and a second substrate 50 are formed on the light emitting diode ED.
When a voltage is applied to the first electrode 34 and the second electrode 40, an electron and a hole are supplied from the first electrode 34 and the second electrode 40 and light is emitted from the emission layer 38. The OLED display device 10 displays a gray level by controlling a current flowing through the light emitting diode ED.
The light emitted from the emission layer 38 may have various paths by transmission, refraction and reflection at interfaces among the emission layer 38, the second electrode 40, the passivation layer 42, the second substrate 50, and the external air.
The path of the light emitted from the emission layer 38 will be illustrated for an exemplary OLED display device where the first electrode 34 is a cathode, the emission layer 38 includes an organic emitting material, the second electrode 40 is a transparent anode, the passivation layer 42 includes an organic insulating material, and the second substrate 50 includes glass. The refraction at the interface between the emission layer 38 and the second electrode 40, the refraction at the interface between the second electrode 40 and the passivation layer 42 and the refraction at the interface between the passivation layer 42 and the second substrate 50 will be ignored.
The light emitted from the emission layer 38 along a normal direction of the second substrate 50 intactly passes through the second electrode 40, the passivation layer 42 and the second substrate 50 without refraction to become a first light L1 which is output by the OLED display device 10 to the exterior.
The light emitted from the emission layer 38 along a direction having a first angle with respect to the normal direction of the second substrate 50 is refracted at the interface between the second substrate 50 and external air to become a second light L2 which is output by the OLED display device 10 to the exterior.
However, the light emitted from the emission layer 38 along a direction having a second angle greater than the first angle with respect to the normal direction of the second substrate 50 is totally reflected at the interface between the second substrate 50 and the external air to become a third light L3 which is not output by the OLED display device 10 to the exterior and the third light L3 returns to the interior. (total internal reflection (TIR) mode)
The light emitted from the emission layer 38 along a direction having a third angle greater than the second angle with respect to the normal direction of the second substrate 50 is totally reflected at the interface between the passivation layer 42 and the second substrate 50 and then is totally reflected at the interface between the second electrode 40 and the passivation layer 42 to become a fourth light L4 which is not output by the OLED display device 10 to the exterior and is dissipated to a lateral surface. In addition, the light emitted from the emission layer 38 along a direction having a fourth angle greater than the third angle with respect to the normal direction of the second substrate 50 is totally reflected at the interface between the emission layer 38 and the second electrode 40 and then is totally reflected at the interface between the emission layer 38 and the first electrode 34 to become a fifth light L5 which is not output by the OLED display device 10 to the exterior and is dissipated to the lateral surface. (waveguide mode)
While the first light L1 and the second light L2 outputted by the OLED display device 10 to the exterior of the OLED display device 10 are used for an image display, the third to fifth lights L3 to L5 are not outputted by the OLED display device 10 and therefore are not used for the image display.
In the OLED display device 10 according to the related art, all of the light emitted from the emission layer 38 is not used for the image display and some of the light is dissipated by the total reflection due to difference in refractive index at the interfaces. Accordingly, the extraction (e.g., output) efficiency of the external light (or external quantum efficiency (EQE)) is less than about 20% due to the light loss.
Specifically, about 40% of the light loss is caused by the waveguide mode where the light is not output to the exterior of the OLED display device 10 and is dissipated to the lateral surface.